Analysis of a spinning polygon wavelength swept laser

TL;DR

This paper reformulates the behavior of spinning polygon wavelength swept lasers in the time domain, revealing they operate as pulsed sources that enable high-speed, low-noise wavelength tuning and providing the first coherence length calculations for such sources.

Contribution

It introduces a novel time domain model for spinning polygon swept lasers and demonstrates their pulsed operation, enabling new insights into their coherence properties.

Findings

Lasers operate as pulsed sources modulating the gain medium.

Pulses hop to longer wavelengths via nonlinear effects.

First coherence length calculations for swept sources.

Abstract

It has been known for quite some time that spinning polygon, and similar, swept lasers used in OCT favor the short to long wavelength sweep direction because of four wave mixing in the gain medium. Here we have reformulated the problem in the time domain and show experimentally and through numerical simulation that these lasers are pulsed. The emitted pulses modulate the gain medium refractive index to red shift the light. Instead of new wavelengths being built up slowly from spontaneous emission, each pulse hops to a longer wavelength by nonlinear means, tracking the tunable filter. This allows high speed, low noise tuning in the blue to red direction. Based on this model, we make the first coherence length calculations for a swept source.